There has long been a need in industry to remove acid gas contaminants from a variety of gas and liquid streams. Common examples include gas and liquid phase hydrocarbon streams, synthesis gas streams and more recently combustion gas streams. A variety of techniques, including absorption into liquids, adsorption onto solids and permeation through membranes, have been developed to accommodate these needs. “Gas Purification” Fifth Edition by Kohl & Nielsen (1997) provides a good overview of techniques involving absorption into liquids and into aqueous amines in particular.
The use of absorption and stripping processes with aqueous solvents such as alkanolamines and promoted potassium carbonate is a known, effective technology for the removal of acid gases from flue gas, natural gas hydrogen, synthesis gas and other gases. Recently, there has been interest in aqueous absorption processes using aqueous amines to remove acid gas contaminants from combustion gas streams. Acid gas contaminants include, but are not limited to carbon dioxide, hydrogen sulfide and carbonyl sulfide. In this context, combustion gas is understood to be the vapor/solid phase combustion products of various fuels (coal, oil, biofuels, natural gas, etc.).
Gas absorption is a process in which soluble components of a gas mixture are dissolved in a liquid. Gas/liquid contact can be counter-current or co-current, with counter-current contact being most commonly practiced. Stripping is essentially the inverse of absorption, as it involves the transfer of volatile components from a liquid mixture to a gas. In a typical carbon dioxide removal process, absorption is used to remove carbon dioxide from a combustion gas, and stripping is subsequently used to regenerate the solvent and capture the carbon dioxide contained in the solvent. Once carbon dioxide is removed from combustion gases and other gases, it can be captured and compressed for use in a number of applications, including sequestration, production of methanol, and tertiary oil recovery.
An absorber designed for counter-current gas liquid contact generally admits the solvent at or near the absorber top. This stream may be referred to as the “lean” solvent. The hydrocarbon stream or combustion gas containing acid gas contaminants enters at or near the absorber bottom. As the solvent travels down the absorber tower, acid gas migrates from the vapor phase to the liquid phase. Column internals, typically packing or trays, provide intimate gas/liquid contact. The purified vapor emerges from the absorber top as product gas. This stream may be referred to as the product gas. The “loaded” or “rich” solvent, which now includes a substantial amount of acid gas, emerges at or near the absorber bottom. During standard operating condition, the lean solvent and the product gas are approximately the same temperature, or the product gas stream may be at a temperature slightly less than the lean solvent stream, at the absorber top.
To effect the regeneration of the absorbent solution, the rich solvent drawn off from the bottom of the absorption column is introduced into the upper half of a stripping column, and the rich solvent is maintained at an elevated temperature at or near its boiling point under pressure. The heat necessary for maintaining the elevated temperature is furnished by reboiling the absorbent solution contained in the stripping column. The reboiling process is effectuated by indirect heat exchange between part of the solution to be regenerated located in the lower half of the stripping column and a hot fluid at appropriate temperature, generally saturated water vapor. In the course of regeneration, the carbon dioxide contained in the rich solvent to be regenerated, maintained at its boiling point, is released and stripped by the vapors of the absorbent solution. Vapor containing the stripped carbon dioxide emerges at the top of the stripping column and is passed through a condenser system which returns to the stripping column the liquid phase resulting from the condensation of the vapors of the absorbent solution, which pass out of the stripping column with the gaseous carbon dioxide. At the bottom of the stripping column, the hot regenerated absorbent solution, also referred to as the lean solvent, is drawn off and recycled to the absorption column after having used part of the heat content of the solution to heat, by indirect heat exchange, the rich solvent to be regenerated, before its introduction into the stripping column.
In simple absorption/stripping, as it is typically practiced in the field, aqueous rich solvent is regenerated at about 100° C. to about 120° C. in a simple, countercurrent, reboiler stripper operated at a single pressure, which is usually at about 1 atm to about 2 atm. The rich solvent feed is preheated by cross-exchange with hot lean solvent product to within about 5° C. to about 30° C. of the stripper bottoms. The overhead vapor is cooled to condense water, which is returned as reflux to the countercurrent stripper. When used for carbon dioxide sequestration and other applications, the product carbon dioxide is compressed to about 100 atm to about 150 atm.
A major problem with existing absorption/stripping processes is that they are very energy intensive, and this is largely because the heat required for the reboiler is significant. Energy for reboiler operations may come from the existing power plant boiler, which decreases energy production, or from a dedicated boiler, which increases capital and operating costs. Therefore, it is important to maximize energy efficiency in the design and operation of these systems.
Hence, there exists a need to reduce the energy necessary to regenerate the loaded aqueous amine stream and improve operational efficiency and cost.